Sputtering, alternatively called physical vapor deposition (PVD), is commonly used to deposit layers of metals and related materials in the fabrication of semiconductor integrated circuits and is also used for depositing coatings of materials onto other types of panels. DC magnetron sputtering from conductive targets is the most prevalently used form of sputtering in the commercial fabrication of integrated circuits. As schematically illustrated in the cross-sectional view of FIG. 1, one type of a DC magnetron sputtering chamber 10 includes a vacuum chamber 12 arranged generally symmetrically about a central axis 14. A vacuum pump system 16 pumps the vacuum chamber 12 to a very low base pressure in the range of about 10−6 Torr or even below. However, an argon gas source 18 connected to the vacuum chamber 12 through a mass flow controller 20 supplies argon into the chamber 12 as a sputter working gas. The argon pressure inside the vacuum chamber 12 is typically held in the low milliTorr range for most materials except copper. A pedestal 22 arranged about the central axis 14 holds a wafer or other substrate to be sputter coated. An unillustrated clamp ring or electrostatic chuck may be used to hold the wafer 24 to the pedestal 22, which is usually heated or water cooled to control the wafer temperature. An electrically grounded (or floating) shield 26 protects the chamber walls and the sides of the pedestal 22 from sputter deposition. A target 28 is arranged in opposition to the pedestal 22 and is vacuum sealed to the vacuum chamber 12 through an insulator 30, which allows the target 28 to be electrically biased with respect to the grounded vacuum chamber 12. At least the front surface of the target 28 is composed of a metallic material to be deposited on the wafer 24, for example, aluminum or titanium.
A DC power supply 32 electrically biases the target 28 to a negative voltage of about −600 VDC with respect to the grounded shield 28 to cause the argon to discharge into a plasma such that the positively charged argon ions are energetically attracted to the negatively biased target 28 and sputter atoms from it, some of which fall upon the wafer 24 and deposit a layer of the target material on it. In reactive ion sputtering, a reactive gas such gas nitrogen or oxygen is additionally admitted to the vacuum chamber 12 to cause the deposition of a metal nitride or oxide. In some applications the pedestal 22 is electrically biased, but in other applications the pedestal 22 is left electrically floating.
The plasma density and hence the sputtering rate can be greatly increased by placing a magnetron 40 in back of the target 28. The magnetron 40, which is an important aspect of the present invention, can assume various shapes and forms. It may include permanent magnets 42, 44 of opposed vertical magnetic polarities, which are covered by respective continuous band-shaped magnetic pole pieces 41, 43. The magnets 42, 44 are typically arranged in a ring shape to form an annular region 46 of a high-density plasma (HDP) adjacent the front face of the target 28. The HDP region 46 results from the magnetic field extending horizontally between neighboring magnetic pole pieces 42, 44 and trapping electrons in front of the target 28, thereby increasing the plasma density. The increased plasma density greatly increases the sputtering of the adjacent region of the target 28. The closed ring shape of the HDP region 46 produces a current loop within the plasma, which increases the discharge efficiency since there are no ends of the loop from which the plasma may leak.
To provide a more uniform target sputtering pattern, the ring-shaped magnetron 40 is typically formed asymmetrically about the central axis 14 and a motor 50 drives a rotary shaft 52 extending along the central axis 14. A plate 54 fixed to and rotated by the rotary shaft 52 supports the magnets 42, 44 and pole pieces 41, 43 so that the magnetron 40 rotates about the central axis 14 and produces an azimuthally uniform time-averaged magnetic field. If the magnetic poles 42, 44 are formed by respective rings of opposed cylindrical permanent magnets, the plate 54 is advantageously formed of a magnetic material to serve as a magnetic yoke to magnetically couple the opposed magnets 42, 44.
Magnetrons of several different designs have been applied to the sputter chamber of FIG. 1. Tepman describes in U.S. Pat. No. 5,320,728 a magnetron that has a flattened kidney shape. For example, as illustrated in the plan view of FIG. 2, a kidney-shaped magnetron 60 includes an outer pole 62 of one magnetic polarity surrounding an inner pole 64 of the other magnetic polarity. A gap 66 of nearly constant width separates the two poles 62, 64 and has a flattened kidney shape. The gap 66 defines an annular band in which the magnetic field between the two poles 62, 64 is approximately horizontal with the sputtering face of the target 28. The kidney-shaped magnetron 60 is relatively large compared to the target 28, having a periphery encompassing a substantial fraction of the area of the target 28. The rotation center 14 of the magnetron 60 falls on or near the inner portion of the inner pole 64. Parker illustrates several variations of the kidney-shaped magnetron in U.S. Pat. No. 5,242,566.
Anderson et al. in U.S. Pat. Nos. 4,995,958 and 6,024,843 describe a cardioid (that is, heart-shaped) magnetron 70 illustrated in plan view in FIG. 3 in which a gap 72 between the two opposed poles forms a symmetric cardioid shape. Anderson et al. have developed a differential equation based upon a constant width for the gap 72. The differential equation for a constant erosion profile has a normalized solution ofθ=√{square root over (r2−1)}−arctan√{square root over (r2−1)},where r=R/R0, R0 is the minimum radius of the plasma track, and R and θ are the polar coordinates of the track. This solution, which is based on several approximations, has several problems. The symmetric cardioid shape has two singular and non-differentiable points 74, 76. The outer singular point 74 can be smoothed out with little effect. The inner singular point 76, however, is not so easily smoothed without incurring substantial effects on the erosion uniformity. Furthermore, the exact solution does not extend any closer to the target center 14 than the inner singular point 76 so that, at least in this approximation, the central area of the target 28 is not scanned or sputtered. Anderson et al. also propose a non-symmetric cardioid shape having some resemblance to a kidney shape. However, Anderson et al. join the ends of two cardioid curves with apparently arbitrary curves of uncertain effect on the erosion profile.
There are two additional problems with the theoretical cardioid shape. It is derived for a constant horizontal magnetic field over the width w of the gap 72. This approximation is fairly accurate for the magnets of Anderson et al., which, as illustrated in the schematic cross-sectional view of FIG. 4, are bar magnets 80 horizontally oriented at the back of the target 28 between magnetic keepers 82, 84, acting as pole faces, to produce a dipole magnetic field 86. As illustrated, the dipole magnetic field 86 has a substantially horizontal orientation over the width w between the pole faces on the front side of the target. However, for a number of reasons, more recent magnetrons have tended to be formed, as illustrated in the schematic cross-sectional view of FIG. 5, from cylindrical magnets 87, 88 of opposed vertical magnetic polarities. Continuous band-shaped pole pieces 90, 92 define annular poles adjacent the target 28. A magnetic back plate 94 supports the magnets 87, 88 and further acts as a magnetic yoke. The structure produces a magnetic field that can be described as a sharpened quadrupole field 96 that is more sharply peaked in the front side of the target 28. As a result, the plasma density is greatest near the centerline of the gap and is not really constant over the width of the gap. The centerline is defined as the mid-point of the gap between the opposed magnetic pole pieces 90, 92. A further problem is that the cardioid shape is based upon a constant erosion profile. While target utilization is important and is maximized by a constant erosion profile, often uniform deposition thickness on the wafer is more important, perhaps at the sacrifice of uniform target erosion. Various attempts have been made to modify the cardioid magnetron for more uniform deposition thickness. However, these techniques are felt to be unsatisfactory and to not allow sufficient flexibility in the design and operation of the magnetron.
The previously described magnetrons have been observed to produce a further deleterious effect of azimuthal sidewall asymmetry. Sidewall asymmetry occurs when one sidewall of a hole is sputter coated with more material than an opposed sidewall. Sidewall asymmetry can cause a number of problems, one of which is the drift of alignment indicia used to align the lithography of one level of the integrated circuit with another. Typical alignment indicia are a series of four trenches arranged in a box shape which are etched into the layer to be sputter coated. The width of the trenches is chosen so that the sputter deposition does not planarize the trench alignment marks. As a result, a surface feature is apparent after sputter deposition that is aligned with the underlying indicia and can be used to align the next level of lithography. For example, the inter-level alignment of a developed photoresist structure is checked with the underlying alignment indicia. The photoresist target may be a square hole that should be centered within the box of the alignment indicia if the photomask is correctly aligned with the alignment indicia. If the alignment is out of specification, the resist is stripped and the photoresist process is repeated. However, it has been found that the alignment indicia shift in both radial and azimuthal directions after a thick aluminum layer is deposited using a standard magnetron. Such shifts would arise if the sputter deposition is not symmetric on opposed sidewalls of the trenches. The asymmetries seem to depend on radial location, the target-to-wafer spacing, and sputter target life. Any inter-level misalignment arising from shifting alignment indicia cannot be detected in the post-exposure check since the photomask was aligned to the shifted indicia.
Radial sidewall asymmetry has long been known and is believed to arise from geometric factors of the magnetron shape and the sputtering pattern. It has been common practice to adjust the mask alignment to compensate for the radial sidewall asymmetry because it is relatively well behaved. Azimuthal sidewall asymmetry presents a more difficult problem. Because the magnetron is being rotated along the entire azimuth to supposedly provide azimuthal uniformity, geometrical factors alone are inadequate to explain the azimuthal asymmetry. The azimuthal asymmetry, particularly at the wafer edge, may be greater than the radial asymmetry. Furthermore, the alignment mark shifts arising from azimuthally asymmetric deposition are very difficult to compensate since they vary over the life time of the target.
Another type of magnetron gaining favor with the larger targets for 300 mm wafers is a spirally arranged 5-track magnetron 100 illustrated in plan view in FIG. 6. It includes a multi-wrapped outer pole 102 of one magnetic polarity which surrounds an inner pole 104 of the opposite polarity. The poles 102, 104 are typically defined by magnetic pole pieces in the illustrated shapes. Cylindrical magnets of opposed polarity underlie the two pole pieces at locations indicated by circular holes 108. A typically constant gap 106 separates the two poles and defines a region of high-density plasma adjacent the front face of the target. The gap 106 is arranged in the form of a spiral loop so that a closed drift current loop is set up in the plasma, an efficient method of maintaining a plasma. In the illustrated embodiment, the center 14 of rotation is placed at the inner end of the inner pole 104. The size of the magnetron 100 is nearly that of the useful area of the target. The magnetron 100 is referred to a 5-track because any path beginning at the target center 14 and proceeding outwardly at any angle within an arc of greater than 180° crosses five tracks of the current loop. As a result, a high density plasma extends over a large portion of the target face despite the magnet poles 102, 104 being separated by a relatively small gap.
The 5-track magnetron 100 however suffers some disadvantages. Relatively small but still significant azimuthally extending erosion grooves still develop in the target. Also, it is desired to operate the sputter chamber at relatively low pressures. It has been found that the minimum chamber pressure to maintain a plasma adjacent a 5-track magnetron is relatively high due to the long plasma track.